Permanent magnet electromagnetic transducer

ABSTRACT

An electromagnetic transducer for actuating the tuning fork of an electronic watch. The transducer is constituted by a magnetic element secured to a tine of a fork which causes the element to vibrate relative to a stationary multiturn coil. The magnetic element is formed by a cylindrical cup having a rodlike permanent magnet core supported coaxially therein to define an annular airgap. The core diminishes in cross section from the cup end to the free end thereof in a nonlinear manner such that the longitudinal profile of the core is continuously curved or nonlinearly tapered to produce a substantially uniform magnetic flux density within the core, the cross section of the coil and its longitudinal profile complementing that of the core.

United States Patent 72] Inventor Dale R. Koehler Westwood, NJ. [21] Appl. No. 45,326 [22] Filed June 1], 1970 [45] Patented Nov. 23, 1971 73] Assignee Bulova Watch Company, Inc.

New York, N.Y.

a [54] PERMANENT MAGNET ELECTROMAGNETIC TRANSDUCER 7 Claims, 7 Drawing Figs. [52] US. Cl 310/25, 335/255 [51 Int. Cl "02k 33/02 [50] Field of Search 310/65, 23, 24, 30, 34, 35, 14, 25; 335/255, 261, 229, 258, 259; 318/122-134; 58/23, 23 MV [56] References Cited UNITED STATES PATENTS 250,463 12/1881 Thomson 335/261 X ABSTRACT: An electromagnetic transducer for actuating the tuning fork of an electronic watch. The transducer is constituted by a magnetic element secured to a tine of a fork which causes the element to vibrate relative to a stationary multiturn coil. The magnetic element is formed by a cylindrical cup having a rodlike permanent magnet core supported coaxially therein to define an annular airgap. The core diminishes in cross section from the cup end to the free end thereof in a nonlinear manner such that the longitudinal profile of the core is continuously curved or nonlinearly tapered to produce a substantially uniform magnetic flux density within the core, the cross section of the coil and its longitudinal profile complementing that of the core.

EL ccrreo/wc PATENTEDNOV m.

SHEET 1 [1F 2 c/eco/f l EYQ INVENTUR. DAL: R KOEHLfQ PERMANENT MAGNET ELECTROMAGNETIC TRANSDUCER BACKGROUND OF THE INVENTION This invention relates generally to electronically controlled timepieces which incorporate electromagnetic transducers for sustaining a tuning fork in vibratory motion, and more particularly to an improved transducer structure that makes possible a substantial reduction in the size of the timepiece.

In the patents to Hetzel Nos. 2,900,786 and 2,971,323, there are disclosed electronic timepieces including a self-sufficient timekeeping standard constituted by a tuning fork whose vibrations are sustained by two electromagnetic transducers operating in conjunction with a battery-energized transistor circuit. The vibratory action of the fork is converted into rotary motion to turn the time-indicating hands of the timepiece.

ln timepieces of the type disclosed in the Hetzel patents, each electromagnetic transducer is associated with a respective tine of the fork, the transducer including a magnetic element attached to the end of the tine and vibrating therewith. The magnetic element on one tine reciprocates with respect to a stationary main drive coil section, and that on the other tine moves back and forth with respect to a stationary minor drive coil section and sensing coil. The two drive coil sections are connected in series to the output circuit of the transistor, while the sensing coil is connected to the input thereof, whereby alternating voltage induced in the sensing coil renders the transistor conductive to produce current pulses in the drive coil sections for magnetically actuating the tines.

When a battery-operated timepiece is to be confined within a small watch casing or in a miniature housing of similar dimensions where space is at a premium, it is essential that the electrical and mechanical efficiency of the system be of exceptionally high order. Otherwise any loss of energy, which in a large-scale device may be negligible, can give rise to serious drawbacks in a more compact structure. It is vital, therefore, that the transducer or transducers which actuate the fork, operate at optimum efficiency, for in this way, even with a battery of small power capacity, it is possible to sustain the vibratory action of the fork for a prolonged period.

Because the sole source of energy for the timepiece is a single-cell miniature battery, any factor which dissipates energy or reduces efficiency not only cuts down the useful battery life, but also creates operating difficulties. In order, therefore, to create a highly compact timepiece, it is important that maximum use be made of all available space and that the transducers which actuate the fork be as small as possible without, however, requiring an undue amount of power.

In the above-identified Hetzel patents, the transducer includes a magnet element formed by a cylindrical cup having a circular cross section, a permanent magnet rod or core of uniform cross section being coaxially mounted within the cylinder to define an annular airgap therein. The stationary drive or sensing coils associated with the magnet element are received within the annular airgap, whereby in operation, the magnetic element reciprocates with respect to the coil. These transducers will hereinafter be referred to as being of the cylindrical magnet rod type.

In the prior patent to Bennett et al. No. 3,22l,l90, there is disclosed an improved transducer arrangement for a timepiece generally of the type disclosed in said Hetzel patents. in this Bennett et al. patent, the permanent magnet is not of uniform cross section throughout its length, but is linearly tapered to assume a frustoconical form, the coil received in the annular airgap being similarly tapered in order to realize the greatest number of turns within the airgap at the position therein of maximum flux density. These transducers will hereinafter be referred to as being of the frustoconical magnetic rod type.

While transducers of the frustoconical magnetic rod type are distinctly superior to those of the cylindrical magnetic rod type, they are not sufficiently efficient to make possible a reduction in the dimensions of a watch to the point where a truly miniature or a ladies size tuning-fork watch movement becomes feasible.

The reason for this is that should one reduce the existing size of a transducer of the cylindrical or frustoconical magnetic rod type, a higher input power would be required to drive the same tuning fork to compensate for the reduction in magnitude of the electromechanical coupling (e.m.c.) factor. The e.m.c. factor is a direct measure of the amount of electrical energy converted to mechanical energy at the transducer interface and therefore the smaller the e.m.c. factor, the larger the electrical power required for a given amount of mechanical power. Hence, with a single-cell battery of the type and size presently used in conjunction with electronic timepiece movements, the cell would be exhausted in a relatively short period. On the other hand, if a larger battery cell is employed, this would be self-defeating, for'the very purpose of reducing the transducer size is to make a smaller watch; hence, if the reduction in transducer size dictates the use of a larger cell, then nothing has been gained. It must be borne in mind that battery size is an important parameter in the overall size of the timepiece, and that the existing dimensions of electronic timepiece battery cells are a limiting factor in producing a smaller watch movement.

In practice, a power requirement for a tuning-fork watch which is in excess of about microwatts cannot be satisfied within the limits ofa commercially acceptable watch volume. With transducer designs of the type heretofore known, should the transducer be made smaller to conserve space, the resultant increase in power requirement would go beyond the tolerable limit due to an increase in the fork amplitude. The improvement resulting from the transducer design of this invention is manifested in both size and power whereby the battery volume may be reduced markedly and therefore the size of the watch movement.

SUMMARY OF THE INVENTION In view'of the foregoing, it is the main object of this invention to provide electromagnetic transducers of exceptional efficiency for actuating a tuning fork in an electronic watch, or for any other appropriate purpose.

More specifically, it is an object of the invention to provide a transducer whose magnetic element includes a permanent magnet rod coaxially mounted in a cylindrical cup, the cross section of the rod diminishing from the cup end to the free end in a nonlinear manner causing the rod to assume a bulletlike shape which gives rise to a substantially uniform magnetic flux density within the rod.

Among the significant advantages of the transducer including a bullet-shaped magnetic rod are the following:

A. Because of its exceptional efficiency, one may reduce the size of the transducer and thereby shrink the size of the associated timepiece movement, and yet operate the timepiece with a material decrease in the power requirement of the movement, thereby effecting 5 decrease in battery size.

B. Because the transducer is markedly more efficient than known transducers, one may scale down the size of the battery and effect a further reduction in volume of the timepiece without raising the power requirement therefor beyond a tolerable limit.

c. Because the improved transducer makes it feasible to shrink the size of the movement, it opens up many new design possibilities, not only for ladies-model watches, but for other miniaturized timing mechanisms.

D. Though the transducer permits a marked reduction in the scale of the movement, it does so without impairing the timing accuracy thereof.

Briefly stated, these objects are attained in a transducer in cluding a magnetic element formed by a cylindrical cup having a magnetic rod supported coaxially therein to define an annular airgap, a multiturn coil being received in the gap. The rod diminishes in cross section from its cup end to its free end in a nonlinear manner, such that the longitudinal profile thereof is continuously curved or otherwise nonlinearly tapered, to produce a substantially uniform magnetic flux density within the rod, the coil having a complementary cross section and longitudinal profile.

OUTLINE OF THE DRAWING For a better understanding of the invention. as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, wherein like components in the several Figures are identified by like reference numerals. In the drawings:

FIG. I is a schematic representation, in perspective of the basic components of an electronic timepiece including a transducer in accordance with the invention;

FIG. 2 is a separate view of the tuning-fork structure showing the transducer partly in section;

FIG. 3 is a side view of the transducer;

FIG. 4 is an enlarged sectional view of the transducer;

FIG. 5 is a diagram of an idealized magnet assembly;

FIG. 6 is a diagram of a prior art magnetic transducer element of the cylindrical rod type; and

FIG. 7 is a diagram of a prior art magnetic transducer of the frustoconical rod type.

DESCRIPTION OF THE INVENTION Referring now to the drawings, and more particularly to FIG. I, the major components of a timepiece including a transducer in accordance with the invention are a timekeeping standard constituted by a tuning fork l0 and an electronic drive circuit 11 therefor, a rotary movement of conventional design including a gear train 12 for turning the hands of the timepiece, and a motion transformer including an index wheel 13 operatively intercoupling fork l0 and the rotary movement, and acting to convert the vibratory action of the fork into rotary motion. The tuning fork has no pivots or bearings and its timekeeping action is therefore independent of the effects of friction.

All of the electrical components of the drive circuit are mounted on the unitized subassembly units or modules F, and F attached to a disc-shaped metallic pillar plate I4 which may be supported within a watch casing of standard design, or within any other type of housing, depending on the use to which the timepiece is put. Electronic circuit 11 is constituted by transistor TR having a base, a collector and an emitter, a resistance-capacitance biasing network R-C, and a bypass capacitor C,, to prevent parasitic oscillations of the circuit. The electronic circuit 11 is energized by a voltage source in the form ofa battery V.

Tuning fork is provided with a pair of flexible tines I5 and 16 interconnected by a relatively inflexible base 17, the base being provided with an upwardly extending stem 18 secured to the pillar plate by suitable screws 19 and 20. The central area of the pillar plate is cut out to permit unobstructed vibration of the tines.

The tuning fork is actuated by means of transducers T, and T Transducer T, is constituted by a magnetic element 21 secured to the free end of tine 15, the element coacting with a stationary drive coil 22 and phase-sensing coil 23. These coils are wound on an open-ended tubular carrier 24 affixed to a subassembly mounting from F, which is secured to pillar plate 14. Coils 22 and 23 may be wound in juxtaposed relation on carrier 24, or the phase-sensing coil 23 may be wound over drive coil 22.

The second transducer T is constituted by a magnetic element 25 secured to the free end of tine l6, and coacting with a drive coil 26 wound on a tubular carrier 27.

The two transducers T, T are of like design, except that an additional coil is provided in transducer T,. The construction and behavior of the transducers are similar to that ofa dynamic speaker of the permanent-magnet type, save that the moving element is the magnet, and not the coil.

FIGS. 2 and 3 show transducer T, in greater detail, and it will be seen that magnetic element 21 is constituted by a cylindrical cup 21a of magnetic material, such as iron, and a permanent-magnet rod 21!) coaxially mounted therein. Magnet 21b, which may be made, for example, of Alnico, is supported on the end wall of the cup to provide a magnetic circuit in which the lines of magnetic flux extend across the annular airgap 210 defined by the inner magnet and the surrounding cylinder. Rod magnet 21b is of diminishing cross section from the cup end to the free end thereof, to produce a longitudinal profile which is continuously curved, for reasons which will be later explained.

As best seen in FIG. 3, cylindrical cup 21a is cut out longitu-' dinally along diametrically opposed planes to form slots 21d and 21s. This effects a substantial reduction in the transducer dimension with relatively little flux leakage. The cutouts act to reduce the space occupied by the cups in depth within the easing, and make possible a more compact construction of the timepiece. The slots also prevent so-called dash-pot" effects resulting from air compression of the magnet-and-cup assembly. Such damping is avoided by the slot openings and also by the openings in the tubular carrier.

It will be seen that fixed carrier 24 for supporting the drive coils 22 and 23 is horn shaped and is dimensioned to complement the tapered magnet 2117. Carrier 24 and the drive coils supported thereon are received within annular gap 21: and are spaced both from the magnet and the surrounding cylinder, whereby the magnetic element is free to reciprocate axially relative to the fixed coil.

In operation, an energizing pulse applied to the drive coils of transducers T, and T, will cause an axial thrust on the associated magnetic element in a direction determined by the polarity of the pulse in relation to the polarization of the permanent magnet and to an extent depending on the energy of the pulse. Since each magnetic element is attached to a tine of the tuning fork, the thrust on the element acts mechanically to excite the fork into vibration.

The vibratory action of the fork and the concomitant movement of the magnetic element includes a back E.M.F. in the drive coil, and in the case of transducer T,, in the phasesensing coil as well. Since the magnetic element reciprocates in accordance with the vibratory action of the tuning fork, the back E.M.F. will take the form of an alternating voltage whose frequency corresponds to that of the tuning fork.

Three functions are served by the transducers. They drive the tuning fork by converting pulses of current delivered to the coils to mechanical pulses. They control the amplitude of the tuning fork by sensing the alternating voltage induced during each cycle; and they control the instant during the cycle when the drive pulse is to be delivered to the coils.

Referring now to FIG. 4, the transducer in accordance with the invention is shown in enlarged form to clarify the relationship of the components thereof and the factors which come into play in optimizing the design.

It will be seen that the permanent magnet rod 21b is coaxially mounted within the cylindrical cup 21a which acts as a high-permeability return member; an annular gap being defined by the interior space between the rod and cup. In order to utilize the magnet to best advantage, there must be a uniform flux density within the magnet, and there must be no leakage flux escaping from the magnetic element. In this connection, reference is made to the article of S. Evershed-Permanent Magnets in Theory and Practice".l. Institute of Electrical Engineers, 13 May I920 (Volume 58,page 797).

Since as one progresses from the base or cup end of the magnet rod to the free end or tip thereof, flux leaks out of the magnet at a rate determined by the reluctance of the magnetic circuit, one must decrease the cross section of the magnet so that the decreased amount of flux in the magnet divided by the decreased cross-sectional area, yields the same flux density at all points in the magnet to attain optimum magnetic performance. Furthermore, the cross section of the rod should diminish to zero at the tip, whereby all of the flux will have been restricted to, or will have emanated in the annular airgap within the cylindrical cup.

Thus the configuration of magnet rod 21!) in FIG. 3 is such as to provide a cross section which diminishes from the cup end to the free end in a nonlinear manner such that the longitudinal profile is continuously curved to produce a rod having a bullet like shape. The cross section at the free end or tip is zero, thereby minimizing flux leakage, and the curvature of the profile is such as to yield the same flux density at all points in the magnet.

The shape of coils 22, 23 which occupy the airgap, complements that of the magnetic element, so that the outer boundary of the coils is cylindrical to conform to the inner surface of the cup, whereas the inner boundary is curved to conform to the curvature of the rod. In this way, full use is made of the available space within the annular gap, the greatest number of coil turns being at the mouth of the gap.

THEORETICAL AND DESIGN CONSIDERATIONS In designing an electromagnetic transducer of the type in question, with a view to optimizing its power performance, one must consider not only the magnetic flux generated by the permanent magnet, but also the volume of available space for the current conductors which are subjected to this magnetic flux.

In the electromagnetic transducer, we therefore encounter an interaction between the field created by the permanent magnet and the field developed by the current-carrying conductors. Hence the factors to be taken into account in seeking to attain optimum performance are the field strength of the permanent magnet, the field strength of the coil operating therewith (other factors remaining the same) and the mass of the magnet elements.

The characteristics of magnetic circuits and magnetic materials of which they are made, are expressed in terms of certain magnetic quantities and units which may be described as follows:

Magnetomotive Force.

In an electromagnet, magnetization is accomplished by means of electric current in windings linked with a magnetic circuit. In an electromagnet of the type in question, the windings are those of the coil placed in the annular gap of a magnetic circuit defined by a permanent rod coaxially supported within a cylindrical cup of high permeability. The total measure of the magnetizing effect of sucha coil is called magnetomotive force F," the units of the force (m.m.f.) being called the gilbert.

Magnetic Flux.

The total measure of the magnetized conditions of a magnetic circuit when acted upon by a magnetomotive force, is called the magnetic flux 1 It is characterized by the fact that a variation in its magnitude gives rise to an E.M.F. in an electric circuit linked with it. The E.M.F. thus induced is at any instant, directly proportional to the time rate of variation of the flux.

Magnetic Reluctance That property of a magnetic circuit which determines the relationship between the magnetic flux and the corresponding m.m.f., is called the magnetic reluctance R of the circuit. It is defined by the following equation:

=E/R. where 1 magnetic flux, maxwells; F=m.m.f., gilberts; and R magnetic reluctance in c.g.s. units. Magnetizing Force.

The m.m.f. acting on a magnetic circuit is distributed along its length in a manner determined by the distribution of the windings and the reluctance of the magnetic circuit. The m.m.f. per unit length along the circuit is called the magnetizing force H," and is defined by the following equation:

H=DF/dl, where H magnetizing force, oersteds; Fm.m.f. gilberts; and l= length, cm. Magnetic Flux Density.

This is the magnetic flux per unit area of a section normal to the flux direction. The c.g.s. unit is called the gauss," and is defined by the following equation:

B=d/dA, where B magnetic flux density, gausses; magnetic flux, maxwells; and A=area, sq. cm.

In order to utilize the permanent magnet in the magnetic circuit to best advantage, it is essential that the magnetic flux density B be uniform within the magnet. It will be shown that in the present magnetic circuit. which involves a cylindrical cup configuration, a diminishing cross section of the magnet from the cup end to the free end is necessary to attain uniform flux density. Since as one progresses from the cup end to the free end of the magnet, flux emanates from the magnet at a rate determined by the reluctance of the magnetic circuit, one must so decrease the cross section of the magnetwhereby the resultant decreased amount of flux divided by the decreased cross-sectional area, yields the same flux density B at all points in the magnet, thereby optimizing the performance of the magnet. Moreover, the cross section must decrease to zero at the tip of the magnet so that all of the flux will have been restricted to or will have emanated in the annular airgap.

We shall begin by considering a transducer having an ideal magnet which produces magnetic flux and thereby generates a magnetic field in a working gap. This is done under ideal circumstances; that is, without any leakage and hence with no loss of flux. In FIG. 5, there are shown tw'o permanent magnets M, and M which are spaced apart to define an airgap in a magnetic circuit completed by a high-permeability, yokeshaped return member. The transducer is to occupy a total volume V which is part magnet, V,,,, and part gap, V,,; hence The flux return volume and the wasted air volume is neglected, a truly ideal situation. The flux conservation statement is as follows:

l m m u uv and the conservation of energy statement is:

realizing that the permeability of air is approximately unity. After multiplying equation l by equation (2 we obtain:

3 B m m u u If we multiply this equation on the right by V and on the left by its equivalent, V,,V,,,, we obtain:

Assuming in our ideal system the absence of flux leakage and that we are able to utilize all of the airgap with ideally placed conductors, then V,,, the volume of the gap, is a measure of the magnetic field generated by the conductors as a result of a fixed current flowing therein. On the further assumption that the size of the conductors and therefore the number of turns per unit area is fixed, the quantity, B l/ or (B,,V,,,, is therefore the interaction term that one seeks to maximize. Let us make the definition.:

BI! u) m m u|( V0 VIII)- We can maximize this quantity with respect to magnet volume by differentiating and setting the result equal to zero That is:

d 1/ m n| m( |u o m I or m oI2- With this result substituted in equation (5), we obtain:

( qmur m nf V02/4- This quantity may be further maximized with respect to the operating conditions of the magnet by designing for peak energy-product point operation. Of course, it is to be realized that there is no guaranty that the two optimization requirements could be simultaneously satisfied. In practice, therefore, the actual design would probably not occur at either V,,,= V or B,,,H,,, maximum, for the magnetic material used. With given transducer volume dimensions as a working constraint, however, one would select a magnetic material to yield the maximum q,,,,,,, therefore achieving the best system possible. The analysis thus far has been concerned with an ideal system and the results are intended to serve as theoretical guidelines.

We shall now proceed to apply these teachings to an electromagnetic transducer system for a mechanical vibrator such as a tuning fork or a reed. Practical considerations dictate a circular cross section for the magnet and conductor coil, but this need not be the case.

The prior art transducer in FIG. 6 as applied to a tuning fork, is of the cylindrical magnet rod type, as shown in the above-identified Hetzel patents, and serves as a good approximation to the ideal system discussed in connection with FIG. 5, in that the annular airgap G between the central magnetic rod R and the cylindrical cup C,., can be considered the working gap. Disposed in the gap is a cylindrical coil S,..

This also applies to the transducer shown in FIG. 7, which is of the prior art frustoconical or linear tapered type shown in the Bennett et al. patent, for most of the magnetic flux spans the volume occupied by the conductors, i.e., relatively little leakage flux is generated. In the tapered rod construction, rod R, coaxially disposed within cup C,. operates in conjunction with a similarly tapered coil 8,.

In discussing the ideal magnet in connection with FIG. 5, no mention was made of the shape of the magnet. However, if there is to be no leakage and ifa single B,,, applies to the entire magnet, then a constant cross section is implied. But in a magnetic element having a cup configuration operating in conjunction with a coaxially mounted magnet rod, a decreasing cross section is desired in the rod so that no flux extends out the end of the cylindrical volume. To this extent, the tapered rod in FIG. 7 is an improvement over the cylindrical rod in FIG. 6.

Of greater importance is that there be a uniform flux density 8 within the magnet to utilize the magnet material to best advantage. This not only requires a decreasing cross-sectional area, but a rate of decrease which is proportional to the rate at which flux flows out of the magnet rod to the wall from the neutral or cup end of the rod to the free end.

Reverting now to H6. 3, which discloses the present invention, and considering a cylindrical hollow shell in the magnet, we can write the equivalent of Ohms Law for this magnetic circuit. The flux in the circuit is:

(7) d= 2y1rdyB=mmfconductance (Hz?) 1 g. (ii/ W w We view this result as the differential equation prescribing the magnet profile. This formulation satisfies the criteria of l no flux out the end of the cylindrical volume, and (2), constant flux density 8 (and H) in the magnetic material.

Solving this differential equation for y, with the boundary conditions that y=o at z=L, and y=r at z=0, leads to the algebraic profile equation:

Having developed the qualitative shape of the magnet, it remains to determine the quantitative dimensions necessary to maximize the transducer efficiency. We can rephrase the original optimization problem for our particular application by writing the differential form of the induced voltage:

11E=dn-dMFdn d ;l /a'z dz/dl, where dn=(RA-y) A dz, where A is a mechanical clearance between magnet and conductors and where A is the wire density (turns per unit area). Now 1;/d1=v (velocity of the coil relative to magnet), and

whence we have,

1 loge /y) The quantity e is again a mechanical clearance at the bottom of the cylindrical cup.

The maximization problem then reduces to determining the value for r the radius of the magnet at its base, which will yield a maximum in E. For the case ofa transducer as part ofa mechanical vibrator, however, there is a further minor refinement necessary. If one examines the equation expressing the power consumed by the vibrator with transducer,

, l E E om 7 2E 276k where R equivalent resistance of vibrator,

E= induced voltage in transducer coil to 211' frequency of vibrator, -r;'= efficiency ofsystem, Q= quality factor of vibrator, m mass of transducer and vibrator, k electromechanical coupling factor; and P= power. one sees that one should really maximize k /m as opposed to k". The quantity k has typically been a measure of the quality of the transducer, but one sees that if consumption of power is the foremost consideration, as it is in timepiece applications. then the quantity k /m is the more important measure.

A step-by-step variation of r,,, from a zero value up to a maximum value dictated by the cylindrical cup dimensions, accompanied by a numerical integration of equation (9) and a calculation of m, will yield a curve for k /m whose maximum determines the choice of r,, for the optimized transducer for the particular magnetic material chosen.

It is understood that these teachings result in both the quantitative and qualitative shape of the magnet and conductor coil to optimize the transducer efficiency, and further, it is appreciated that approximations to the curve of equation (8) such as multitaper configurations in which the cross section of the rod diminishes in a series of tapered steps, whereby the flux density Bis approximately uniform, can yield almost optimum results. These teachings therefore make possible the fabrication of very physically small transducers which can operate on a minimum of power.

COMPARATIVE IMPROVEMENT It has been determined, for a particular small transducer size applicable to a ladies tuning-fork watch movement, that a transducer in accordance with the invention is strikingly superior in its performance characteristics to known types of transducers of the cylindrical magnet rod and the frustoconical magnetic rod type. The following table shows the approximate comparison for these three types of transducers, having the same magnetic-element cup sizes but different magnet rod shapes for a particular transducer size applicable to a ladies watch movement of the type envisioned.

Power to Drive Fork nil/t in arbitrary units Transducer Magnet Shape Cylindrical Frustnconical Bullet-shaped (Present Invention) l.ll

This table shows clearly the significance of the present invention. It indicates a 7:1 reduction in power input required to operate a tuning fork as a result of employing the optimum transducer design of this invention, in comparison with the more conventional cylindrical magnet shape. This, obviously, permits one to design a tuning-fork transducer drive system with substantially smaller size (which would otherwise require much more power because of reduced efficiency), yet requiring less battery power and therefore a smaller battery, resulting in a much smaller watch movement complete with selfcontained battery.

It should also be appreciated that there are many areas of application for electromagnetic transducers other than the Table ll Relative Effectivity Transducer Magnet Shape k in arbitrary units Cylindrical Frustoconicul Bullet-shaped (Present Invention) While there has been shown and described a preferred embodiment of electromagnetic transducer in accordance with the invention, it will be appreciated that many changes and modifications may be made therein, without, however, departing from the essential spirit of the invention.

lclaim:

1. An electromagnetic transducer comprising:

a. A magnetic element constituted by a cylindrical cup having a permanent-magnet rod fixedly supported coaxially therein to define an annular airgap, said rod having a cross section which diminishes from the cup end of the rod to the free end thereofin a nonlinear manner to yield substantially the same flux density at all points in the rod. and

b. a multiturn coil received within said annular airgap, the cross section of the coil substantially complementing that of the rod, said magnetic element being movable relative to said coil.

2. A transducer as set forth in claim I, wherein said rod has a longitudinal profile which is continuously curved, the free end of the rod having an approximately zero cross section.

3. A transducer as set forth in claim 1. wherein said rod diminishes in cross section in a series of tapered steps.

4. A transducer as set forth in claim 1, wherein said coil is stationary and said magnetic element is mounted on a vibratory member to reciprocate the element relative to the coil.

5. A transducer as set forth in claim 1, wherein said cylindrical cup is longitudinally slotted on diametrically opposed sides.

6. A transducer as set forth in claim 1, wherein said rod is formed ofa material having a high value of residual induction and coercive force. and said cup is formed ofa material having high flux permeability.

7. An electronic watch provided with a tuning fork, means to convert the vibratory action of the fork into rotary motion to drive the gearworks of the watch, and means to sustain the fork in vibration, said last-named means comprising:

a. an electromagnetic transducer including:

1. a magnetic element constituted by a cylindrical cup having a permanent-magnet rod fixedly supported coaxially therein to define an annular airgap, said element being mounted on one tine of the fork to vibrate therewith, said rod having a cross section which diminishes from the cup end to the free end thereofin a nonlinear manner to yield substantially the same flux density at all points in the rod, and

2. a multiturn drive coil received within said annular airgap, the cross section of the coil substantially complementing that of the rod, said coil being mounted at a stationary position in said watch, and

b. means to apply electrical pulses to said coil to sustain said fork in vibration. 

1. An electromagnetic transducer comprising: a. A magnetic element constituted by a cylindrical cup having a permanent-magnet rod fixedly supported coaxially therein to define an annular airgap, said rod having a cross section which diminishes from the cup end of the rod to the free end thereof in a nonlinear manner to yield substantially the same flux density at all points in the rod, and b. a multiturn coil received within said annular airgap, the cross section of the coil substantially complementing that of the rod, said magnetic element being movable relative to said coil.
 2. a multiturn drive coil received within said annular airgap, the cross section of the coil substantially complementing that of the rod, said coil being mounted at a stationary position in said watch, and b. means to apply electrical pulses to said coil to sustain said fork in vibration.
 2. A transducer as set forth in claim 1, wherein said rod has a longitudinal profile which is continuously curved, the free end of the rod having an approximately zero cross section.
 3. A transducer as set forth in claim 1, wherein said rod diminishes in cross section in a series of tapered steps.
 4. A transducer as set forth in claim 1, wherein said coil is stationary and said magnetic element is mounted on a vibratory member to reciprocate the element relative to the coil.
 5. A transducer as set forth in claim 1, wherein said cylindrical cup is longitudinally slotted on diametrically opposed sides.
 6. A transducer as set forth in claim 1, wherein said rod is formed of a material having a high value of residual induction and coercive force, and said cup is formed of a material having high flux permeability.
 7. An electronic watch provided with a tuning fork, means to convert the vibratory action of the fork into rotary motion to drive the gearworks of the watch, and means to sustain the fork in vibration, said last-named means comprising: a. an electromagnetic transducer including: 